Vesicular transport with emphasis on exocytosis

The eukaryotic cell is compartmentalized by a series of vesicular organelles which constitute the endocytic and exocytic transport pathways. Each vesicular compartment has distinct sets of membrane proteins, structures and functions. Despite continuous vesicular transport, each vesicular compartment maintains its structure and function by use of retention and retrieval signal for its own resident proteins. Proteins in transit along the endocytic and exocytic pathway are transported without admixing with cytoplasmic constituents by successive steps of budding from the donor vesicles, formation of intermediate transport vesicles, transport, targeting to and fusion with acceptor vesicles. Specificity and fidelity of the vesicular transport are conferred by vesicular membrane proteins and small molecular weight GTP-binding proteins of the Rab subfamily. Proteins for export are packaged into specific vesicles for their final destinations. Insertion into and retrieval from the plasma membrane of transport proteins in response to cellular stimulus are a new paradigm of cellular regulatory mechanism. Secretion of neurotransmitters, hormones and enzymes by exocytosis involves a complex set of cytosolic proteins, G-proteins, proteins on the secretory granule membrane and plasma membrane. Much progress has been recently made in identifying proteins and factors involved in the exocytosis. But the molecular interactions among identified proteins and regulatory factors are unknown and remain to be elucidated. Finally our chemiosmotic hypothesis which involves the H+ electrochemical gradient across the secretory granule membrane generated by an ATP-dependent electrogenic H(+)-ATPase as the potential driving force for fusion and release of granule contents will be discussed.